The cortico-basal ganglia system is a complex network involved in motivation and reward. Medium spiny projection neurons (MSNs) within the striatum serve as the primary relay station of this circuit, and guide behavioral output through two divergent pathways. Striatal MSNs that contain the neuropeptides dynorphin (DYN) and substance P are part of the direct pathway whereas striatal neurons that contain the neuropeptide enkephalin (ENK) are part of the indirect pathway. Repeated exposure to drugs of abuse can produce dysregulation in the cortico-basal ganglia circuit, in part through aberrant plasticity at sub-cortical structures, such as the striatum, as well as through a loss of top-down control from glutamatergic afferent systems. Direct and indirect striatal MSNs have recently been shown to play important, but distinct, roles in behaviors produced by psychostimulant drugs. However, little is known regarding the roles of these distinct cell populations, as well as their afferent connections, in models that produce patterns of compulsive drug-taking and drug-seeking that are characteristic of addiction, such as prolonged access drug self-administration. In addition, how signaling activity in these striatal cell populations changes following compulsive drug use, as well as regulates addiction-like behavior is not well understood. The overall aim of the proposed experiments is to use novel viral vectors expressing engineered DREADD (Designer Receptor Exclusively Activated by Designer Drug) receptors to define the role of direct and indirect pathway MSNs, as well as glutamatergic afferents, in compulsive drug-taking and drug-seeking behaviors. In addition, how compulsive patterns of drug use alter intracellular signaling activity in striatal cell populations will be examined. The central hypothesis of this proposal is that repeated drug use produces glutamate-dependent modifications to G-protein dependent signaling cascades in the striatum in a subset of individuals, leading to compulsive, addiction-like behavior (defined by high motivation to take drugs, and drug-seeking during periods of drug unavailability and during punishment). Specifically, we propose that ERK/MAPK signaling cascades are enhanced in direct pathway MSNs and suppressed in indirect pathway MSNs, which shifts the balance of these pathways towards the 'go' drive, thereby increase drug-taking and drug-seeking. This work has the potential to identify new targets for treatment (i.e., specific cell populations or afferent projections) as well as new types of treatments that may reverse addictive behaviors, such as DREADD receptor modulation of intracellular signaling cascades.